7.8.4.5 Pattern based caching

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An application has three options for telling PECL/mysqlnd_qc whether a particular statement shall be used. The most basic approach is to cache all statements by setting mysqlnd_qc.cache_by_default = 1. This approach is often of little practical value. But it enables users to make a quick estimation about the maximum performance gains from caching. An application designed to use a cache may be able to prefix selected statements with the appropriate SQL hints. However, altering an applications source code may not always be possible or desired, for example, to avoid problems with software updates. Therefore, PECL/mysqlnd_qc allows setting a callback which decides if a query is to be cached.

The callback is installed with the mysqlnd_qc_set_is_select function. The callback is given the statement string of every statement inspected by the plugin. Then, the callback can decide whether to cache the function. The callback is supposed to return FALSE if the statement shall not be cached. A return value of TRUE makes the plugin try to add the statement into the cache. The cache entry will be given the default TTL ( mysqlnd_qc.ttl). If the callback returns a numerical value it is used as the TTL instead of the global default.

Example 7.314 Setting a callback with mysqlnd_qc_set_is_select

mysqlnd_qc.enable_qc=1
mysqlnd_qc.collect_statistics=1

<?php
/* callback which decides if query is cached */
function is_select($query) {
    static $patterns = array(
      /* true - use default from mysqlnd_qc.ttl */
      "@SELECT\s+.*\s+FROM\s+test@ismU" => true,
      /* 3 - use TTL = 3 seconds */
      "@SELECT\s+.*\s+FROM\s+news@ismU" => 3
    );
    /* check if query does match pattern */
    foreach ($patterns as $pattern => $ttl) {
        if (preg_match($pattern, $query)) {
            printf("is_select(%45s): cache\n", $query);
            return $ttl;
        }
    }
    printf("is_select(%45s): do not cache\n", $query);
    return false;
}
/* install callback */
mysqlnd_qc_set_is_select("is_select");
/* Connect, create and populate test table */
$mysqli = new mysqli("host", "user", "password", "schema", "port", "socket");
$mysqli->query("DROP TABLE IF EXISTS test");
$mysqli->query("CREATE TABLE test(id INT)");
$mysqli->query("INSERT INTO test(id) VALUES (1), (2), (3)");
/* cache put */
$mysqli->query("SELECT id FROM test WHERE id = 1");
/* cache hit */
$mysqli->query("SELECT id FROM test WHERE id = 1");
/* cache put */
$mysqli->query("SELECT * FROM test");
$stats = mysqlnd_qc_get_core_stats();
printf("Cache put: %d\n", $stats['cache_put']);
printf("Cache hit: %d\n", $stats['cache_hit']);
?>

The above examples will output something similar to:

is_select(                    DROP TABLE IF EXISTS test): do not cache
is_select(                    CREATE TABLE test(id INT)): do not cache
is_select(    INSERT INTO test(id) VALUES (1), (2), (3)): do not cache
is_select(             SELECT id FROM test WHERE id = 1): cache
is_select(             SELECT id FROM test WHERE id = 1): cache
is_select(                           SELECT * FROM test): cache
Cache put: 2
Cache hit: 1

The examples callback tests if a statement string matches a pattern. If this is the case, it either returns TRUE to cache the statement using the global default TTL or an alternative TTL.

To minimize application changes the callback can put into and registered in an auto prepend file.